Blood clots that form in the lower part of the body may migrate to the heart and may be subsequently pumped to the lungs. Small clots can be absorbed by the body without adverse effect. However, larger clots (e.g., on the order of 3 mm in diameter and 10-30 cm in length) can interfere with the oxygenation of blood and can possibly cause shock or sudden death.
Many transvenous filtering devices have been developed for installation in the vena cava to prevent especially large clots from reaching the lungs. These filters have fine wires positioned in the blood flow to catch and hold clots for effective lysing in the blood stream. Some of these devices are inserted into the vena cava by dissecting the internal jugular vein in the neck or the femoral vein in the groin, inserting a metallic capsule containing a filtering device to the proper position in the vena cava, and releasing the filtering device into the vena cava. More recently, filters have been designed for percutaneous introduction into the vasculature.
U.S. Pat. No. 5,344,427 discloses a filter including extending wire portions folded in the shape of a hairpin into a plurality of resilient triangular fingers. The particular shape of the triangular fingers is meant to enable the wire portions to be moved together easily, for example, for radially contracting the filter for delivery within a vessel. With such a design, however, the finger portions can "bunch up" in a non-symmetrical pattern around the circumference of the device, and can cause improper spacing of the filter element within a vessel.